Influence of electrical discharge machining on the reciprocating sliding wear response of WC-Co cemented carbides

Influence of electrical discharge machining on the reciprocating sliding wear response of WC-Co cemented carbides

Wear 266 (2009) 84–95 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Influence of electrical discharg...

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Wear 266 (2009) 84–95

Contents lists available at ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

Influence of electrical discharge machining on the reciprocating sliding wear response of WC-Co cemented carbides K. Bonny a,∗ , P. De Baets a , W. Ost a , J. Vleugels b , S. Huang b , B. Lauwers c , W. Liu c a

Ghent University (UGent), Department of Mechanical Construction & Production, IR04, Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium Catholic University Leuven (K.U. Leuven), Department of Metallurgy & Materials Engineering, MTM, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium c Catholic University Leuven (K.U. Leuven), Department of Mechanical Engineering, PMA, Celestijnenlaan 300 B, B-3001 Leuven, Belgium b

a r t i c l e

i n f o

Article history: Received 28 November 2007 Received in revised form 26 April 2008 Accepted 27 May 2008 Available online 10 July 2008 Keywords: Cemented carbide Wire-EDM Surface integrity Dry friction Reciprocating sliding wear Pin-on-plate

a b s t r a c t A number of commercially available WC-Co-based cemented carbides with 6 up to 12 wt.% Co were machined and surface finished by grinding as well as by wire electrical discharge machining (EDM) in demineralised water through a number of consecutive gradually finer EDM regimes. Comparative dry reciprocating sliding wear experiments on both wire-EDM and ground samples against WC-Co pins were conducted, using a pin-on-plate test rig, in order to investigate the influence of the EDM process on the tribological behavior. The worn surfaces of the investigated cemented carbides were scanned topographically and characterised by scanning electron microscopy (SEM). The post-mortem obtained wear volumes were compared to the online measured wear in order to determine correlations between wear volume and wear rate on the one hand and sliding distance on the other hand. The experimental results revealed a profound influence of surface finish conditions and distinctive EDM regimes on the wear behavior of WC-Co cemented carbides. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Based on economic reasons but especially today on the basis of ecological considerations as well, there is a rising need for an adequate limitation of wear and corrosion damage of machines and construction tools with attention to the efficient application of scarce materials and resources such as energy. In this way, there is an obvious industrial demand for wear resistant materials to be applied under heavy tribological circumstances and preferably without lubrication as for instance for tools (chisels, cutting tools, metal forming dies, punches, etc.) and various machine parts. Furthermore, in the fields of aerospace and automobile, high performance materials with the properties of ultra-hard, erosion/friction-resistant and high-temperatureresistance are more and more claimed and applied. However, a significant drawback of some of these advanced materials, such as engineering ceramics, is their relatively high coefficient of friction in dry contact conditions, involving heat development and energy loss. The high hardness of engineering ceramics also renders them intrinsically difficult to shape and finish by conventional methods.

∗ Corresponding author. Tel.: +32 4 85 52 30 04; fax: +32 9 2 64 32 95. E-mail addresses: [email protected], [email protected] (K. Bonny). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.05.009

Profile grinding with super hard grinding grains (CBN, diamond) is almost the only possibility to shape those hard materials, but the shapes that can be generated by grinding cannot be intricate. New manufacturing techniques are required to machine these materials into complex shapes as often desired (e.g. intricate small punches for blanking contactor blades for multi-polar electric switches and connectors). Electro-discharge machining (EDM) is one of the nonconventional manufacturing processes that allow to produce complicated shapes in electrically conductive materials irrespective of their strength or hardness: i.e., the strength and hardness is no limitation to the machinability, provided the material is electrically conductive, which is not the case generally for engineering ceramics. Today electro-erosion is widely used to machine cermets [1]. More specifically, EDM has successfully proven to be feasible for manufacturing WC-Co cemented carbides [2,3]. However, difficulties also arise with respect to the control of surface finish and the influence the machining parameters may have on final properties such as strength, hardness and fracture toughness [4–6] as well as friction and wear characteristics [7–10]. EDM damage is given by residual stresses and cracks produced within the thermally affected zone (recast layer and adjacent regions) beneath the shaped surface and its degrading effect is strongly dependent upon external EDM parameters such as type of power supply, machining variables and operator experience [11–14].

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Table 1 Chemical, physical, mechanical and microstructural properties of WC-Co cemented carbides Hardmetal grade

WC10Co

WC12Co(V)

WC12Co(Cr)

WC10Co(Cr/V)

WC6Co(Cr/V)

Co binder content (wt.%) WC grain growth inhibitor Density (g/cm3 ) Thermal conductivity* (W m−1 K−1 ) Vickers hardness, HV10 (kgf/mm2 ) Fracture toughness, KIC(30 kgf) (MPa m1/2 ) E-modulus (GPa) Compressive strength* (GPa) Average WC grain size, dav (␮m) WC grain size, d50 (␮m) WC grain size, d90 (␮m) WC grain size, d95 (␮m)

10 None 14.33 105 1149 ± 10 >15.5 578 ± 6 4.2 2.2 1.8 4.2 6.0

12 VC 14.08 95 1286 ± 8 15.4 ± 0.5 563 ± 2 4.9 0.9 0.7 1.5 1.8

12 Cr3 C2 14.01 95 1306 ± 5 15.5 ± 0.6 546 ± 2 4.9 0.9 0.8 1.7 2.1

10 Cr3 C2 /VC 14.23 85 1685 ± 38 9.7 ± 0.2 541 ± 4 6.6 0.3 0.3 0.6 0.7

6 Cr3 C2 /VC 14.62 90 1913 ± 13 8.8 ± 0.2 609 ± 4 7.2 0.6 0.5 1.0 1.2

*

Data specified by manufacturer.

This paper aims to elucidate the influence of different EDM cutting regimes on the wear behavior of a number of commercially available WC-Co cemented carbide grades sliding reciprocally against WC-Co cemented carbide pins. Correlations between the wear volume and wear rate on the one hand and surface conditions associated with sequential EDM of commercial WC-Co cemented carbides on the other hand were determined. 2. Experimental 2.1. WC-Co cemented carbides The chemical, physical, mechanical and microstructural properties of the distinctive cemented carbide grades investigated are listed in Table 1. The WC10Co, WC12Co(V), WC12Co(Cr), WC10Co(Cr/V) and WC6Co(Cr/V) cemented carbide grades are CERATIZIT grades GC32, GC20, GC20CR, MG18 and MG12, respectively. The HV10 Vickers hardness was measured with an indentation load of 10 kgf (Model FV-700, Future-Tech Corp., Tokyo, Japan). The fracture toughness KIC(30 kgf) was obtained by the Vickers indentation technique, based on crack length measurements of the radial crack pattern produced by Vickers HV30 indentations. The KIC values were calculated according to the Shetty formula, which is given by Eq. (1):



KIC = 0.0889

HV

P 4l

(1)

with HV , the Vickers hardness, P, the indentation load (N) and l, the total crack length (m), which is defined as the radial crack length (c) minus half the indentation diagonal length (a). The Young’s modulus E was measured by the resonance frequency method. The resonance frequency was obtained on a Grindo-Sonic (J.W. Lemmens, Elektronika N.V. Leuven, Belgium), by means of the impulse excitation method (ASTM E 1876–99). The electrical resistivity was measured by a four terminal method on a Resistomat (TYP 2302 Burster, Gernsbach, Germany). The grain size distribution of the cemented carbide grades was acquired using computer image analysis software according to the linear intercept method. At least 1000 grains were measured for each grade. The WC10Co grade exhibits the coarsest WC grain structure, with 50% of the grains being smaller than 1.8 ␮m and 95% being smaller than 6.0 ␮m. The WC10Co(Cr/V) grade has the finest microstructure, with 95% of the grains smaller than 0.7 ␮m. 2.2. Surface finishing The above mentioned cemented carbides were manufactured and surface finished by (i) wire-EDM (ROBOFIL 2030SI, Charmilles Technologies, Switzerland) in demineralised water (dielectric con-

ductivity 5 ␮S/cm), with a CuZn37 wire electrode (diameter 0.25 mm, tensile strength 500 MPa), or (ii) grinding (JF415DS, Jung, Göppingen, Germany), with a diamond grinding wheel (type MD4075B55, Wendt Boart, Brussels, Belgium). The wire-EDM process was performed using one rough cutting, with high spark thermal energy and a concomitant high material removal rate, followed by several consecutive finish cuts, with globally decreasing energy input and pulse duration, in order to improve surface quality. The rough EDM regime, i.e., E3, and 3 finer surface variants, i.e., E8, E21 and E23, were selected for this investigation. The corresponding wire-EDM generator settings applied on the WC-Co cemented carbide grades are listed in Table 2. Surface conditions of ground and wire-EDM’ed hardmetal grades were analyzed, both before and after wear testing, by scanning electron microscopy (SEM, XL30FEG, FEI, Eindhoven) and surface topography (Somicronic® EMS Surfascan 3D, needle type ST305).

2.3. Wear testing The dry reciprocating sliding friction and wear response of wire-EDM’ed and ground WC-Co cemented carbide plates (width w = 38 mm, length l = 58 mm and thickness t = 4 mm) against WC6Co(Cr/V) cemented carbide pins (length l = 22 mm and diameter d = 7.9 mm) was evaluated using a high frequency tribometer (Plint TE77) [15,16], in an air-conditioned atmosphere of 23 ± 1 ◦ C and a relative humidity of 60 ± 1%, in conformity with ASTM G133 [17]. A schematic illustration of the pin and plate test specimen geometry in the applied wear testing system is presented in Fig. 1. The pin was a hemisphere of which the average rounding radius R and surface roughness Ra and Rt were determined to be 4.08 mm, 0.35 and 2.68 ␮m, respectively. Four surface conditions were related to sequential EDM, i.e., one rough (E3) and 3 finish regimes (E8, E21, and E23), whereas the other one was attained by conventional grinding.

Table 2 Wire-EDM parameters applied during rough (E3) and 3 finer (E8–E23) EDM cutting regimes EDM regime Open voltage (V) Pulse duration, te ((s) Pulse interval, t0 ((s) Maximum speed (mm/min) Reference servo voltage Aj (V) Pulse ignition height IAL (A) Flushing pressure (bar) Wire tension (N) Wire winding speed (m/min)

E3 80 1.2 8.3 14.5 50 8 6.5 11 8

E8 80 1 10 14.5 13.2 16 0 16 6.8

E21

E23

140 1 4 6.1 6 4.5 0 10 6.8

140 1 4 8 0 2.5 0 10 4.8

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Fig. 1. Schematic outline of the pin-on-plate wear testing setup.

Contact loads were varied from 15 N up to 35 N. The stroke length of the oscillating motion was 15 mm. A sliding velocity of 0.3 m/s was applied. The test duration was associated with a sliding distance of 10 km, allowing post-mortem wear volumes to be compared. Before each test, the specimens were cleaned ultrasonically with acetone. 3. Results and discussion 3.1. Surface roughness The surface topographies for the WC12Co(Cr) grade with distinctive surface finishing conditions are compared in Fig. 2. It is worth noting that the surface topographies for all WC-Co cemented carbides were found to resemble those presented in Fig. 2, both after grinding and after consecutive EDM regimes. Based on surface scanning measurements, the Ra and Rt surface roughness for the distinctive cemented carbides was derived, Table 3. In all cases, Ra and Rt are noticed to exhibit a similar trend during the EDM process. It is obvious that the surface roughness is progressively reduced when consecutively finer EDM cuttings were applied. For the WC12Co(Cr) samples, for instance, the Rt -value decreased from 16.08 ␮m after rough cutting to 1.02 ␮m after the final finish cut. No large differences are encountered in the obtained Ra - and Rt -values amongst the WC-Co grades. Under equal setup and identical series of EDM parameters, the WC10Co specimens obtain the smoothest surfaces, whereas the highest roughness values are encountered with the WC6Co(Cr/V) cemented carbides. These differences should be related to the grain size distribution of the WC-Co cemented carbides. 3.2. Wear characteristics Two methods were utilized in order to quantify wear. Firstly, online measurements of wear depth (d) resulting from pins penetrating the counter plate samples are obtained by an inductive displacement transducer. Simultaneously, a wear ‘depth’ rate (kd ) was derived from the ratio of penetration depth and the product of the applied load (FN ) and sliding distance (s), Eq. (2): kd (s) =

d(s) (m N−1 m−1 ) FN · s

(2)

The second procedure comprises the post-mortem assessment of the wear scar dimensions, i.e., depth, length, width and volume, using topographical scanning equipment. Vertical displacement and penetration rate curves, measured during reciprocative pin-on-flat wear experiments for the

wire-EDM’ed and ground WC12Co(Cr) flat/WC-6Co(Cr/V) pin combinations, are plotted as function of the sliding distance in Fig. 3. The axes are presented on a logarithmic scale, allowing to zoom in on the initial stage, and as well as on linear scale, in order to focus on the wear behavior after running-in. Each curve is an average of at least two wear experiments performed under identical conditions. Error bars indicating the extent of the variations were excluded in order to make the figures better readable. For each sliding wear experiment, a new WC6Co(Cr/V) pin was used in order to ensure identical initial surface conditions. It should be noted that both combined pin/plate penetration depth and penetration rate curves for all ground and wire-EDM’ed WC-Co cemented carbides and surface conditions were similar to these presented in Fig. 3. Some numerical data are provided in Table 4, in which penetration depth and penetration rate values for all tested tribopairs are compared as function of sliding distance and surface finishing conditions. Within the full wear path range, the lowest penetration depths and concomitant wear rates were recorded for the WC10Co(Cr/V) cemented carbide, whereas the WC10Co grade exhibits the lowest wear resistance. This trend is in full agreement with previous research [7,8], and is related to the mutual differences in hardness, i.e., WC grain size and Co binder content, between the distinctive grades. In all cases, the penetration depth is noticed to increase abruptly during the first sliding cycles, owing to the quickly growing contact surface area, in correspondence with a maximum value for the wear depth rate. After the initial stage, which is mainly determined by strong asperity interaction (fragmentation and deformation) and wearing down asperities, penetration depth increases further at a continuously reducing rate, which appears to decrease exponentially with sliding distance after the running-in wear. For example, based on the results presented in Fig. 3, following correlation between penetration rate and wear path were derived by curve fitting, Eq. (3): kd,WC12Co(Cr)−E23 = 0.2033 · s−0.8084

(s > 0.3 km),

2 RWC12Co(Cr)−E23 = 0.9985

(3)

with kd,WC12Co(Cr)–E23 the combined penetration rate for the WC12Co(Cr) grade with finest EDM regime (E23) sliding against WC6Co(Cr/V) under a 15 N contact load, and s the sliding distance beyond 0.3 km. The R2 -value indicates that the equation perfectly matches the experimental results. The effect of the wire-EDM process and the distinctive EDM regimes on the wear performance is quite pronounced. The best wear resistance is encountered with the ground cemented carbides, whereas the rough cut EDM specimens exhibit the largest penetration depth levels and penetration rates. Furthermore, the wear damage is noticed to decrease with finer executed EDM, down to values nearby those recorded for the equivalent ground cemented carbide grades. The differences in wear damage between the distinctive surface finishes are attributed to the wire-EDM induced heat-affected zone (HAZ) and the concomitant surface roughness, Figs. 4–6. Indeed, in case of ground surfaces, the pin slides directly against the base material of WC-Co cemented carbide, for which the wear process reaches a steady state regime within a wear path range of less than 1 km. This steady state is characterised by a constant slope in penetration depth as function of wear path, indicated by a thin line in Fig. 3(b). After rough cut EDM machining (regime E3), on the other hand, the WC-Co cemented carbide is covered by a ca. 20 ␮m thick recast layer, which is obviously less wear resistant compared to the bulk material. Therefore, both penetration depth and penetration rate exhibit the largest values for rough cut WC-Co cemented

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Fig. 2. Surface topographies for WC12Co(Cr) grade: (a) E3, (b) E8, (c) E21, (d) E23 and (e) grinding. Table 3 Ra and Rt roughness for the rough (E3) and smoother EDM regimes (E8, E21, and E23) and grinding (measured in perpendicular direction) of the cemented carbides Grade

WC10Co

Surface finish

Ra (␮m)

Rt (␮m)

Ra (␮m)

WC12Co(V) Rt (␮m)

Ra (␮m)

WC12Co(Cr) Rt (␮m)

Ra (␮m)

WC10Co(Cr/V) Rt (␮m)

Ra (␮m)

WC6Co(Cr/V) Rt (␮m)

E3 E8 E21 E23 Grinding ⊥

2.08 1.07 0.24 0.15 0.25

15.18 6.62 2.16 1.02 2.04

2.31 1.3 0.26 0.18 0.27

15.84 7.16 2.38 1.04 2.08

2.37 1.29 0.24 0.16 0.19

16.08 8.67 2.99 1.02 1.74

2.34 1.22 0.24 0.17 0.26

17.36 6.81 2.73 1.08 2.25

2.08 1.00 0.37 0.24 0.22

17.02 5.95 3.05 1.29 1.83

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Fig. 3. Penetration depth (a and b) and penetration rate (c) as function of surface conditions for WC12Co(Cr)–WC6Co(Cr/V) sliding pairs (v = 0.3 m/s, FN = 15 N). Table 4 Penetration depth and penetration rate as function of sliding distance for the wire-EDM’ed and ground cemented carbide flat/WC6Co(Cr/V) pin sliding combinations (FN = 15 N, v = 0.3 m/s) Grade

kd (10−9 ·N−1 )

Surface finish

d (␮m)

Distance (km)

0.015

1

4

10

0.015

1

4

10

WC10Co

E3 E8 E21 E23 Grinding

4.6 2.7 1.4 1.0 0.7

7.6 6.1 4.1 3.4 2.2

11.7 9.4 5.9 5.2 3.3

17.4 12.5 7.5 7.0 4.8

20.7 12.1 6.1 4.3 3.3

0.50 0.41 0.27 0.22 0.15

0.19 0.16 0.10 0.087 0.056

0.11 0.084 0.048 0.047 0.032

WC12Co(V)

E3 E8 E21 E23 Grinding

5.0 3.9 2.8 1.8 0.8

6.9 6.1 3.4 3.1 1.7

10.2 8.5 4.5 3.7 2.5

15.0 11.1 5.5 4.8 3.3

22.4 17.4 12.5 7.9 3.5

0.46 0.41 0.23 0.20 0.11

0.17 0.14 0.074 0.062 0.041

0.10 0.075 0.037 0.032 0.022

WC12Co(Cr)

E3 E8 E21 E23 Grinding

4.6 4.4 1.5 1.2 0.8

6.9 5.5 3.2 3.1 1.8

7.5 6.0 4.3 4.0 2.7

9.1 7.0 5.3 5.0 3.8

17.9 14.2 6.4 5.3 3.5

0.56 0.39 0.22 0.21 0.12

0.18 0.12 0.082 0.075 0.045

0.099 0.069 0.047 0.042 0.026

WC10Co(Cr/V)

E3 E8 E21 E23 Grinding

3.8 3.6 2.6 1.8 0.4

5.5 5.1 2.8 1.9 1.0

8.3 7.3 3.2 3.0 1.3

12.7 10.2 4.6 4.1 2.1

16.8 15.7 9.7 6.9 1.8

0.36 0.35 0.18 0.13 0.07

0.14 0.12 0.053 0.049 0.022

0.085 0.068 0.031 0.028 0.014

WC6Co(Cr/V)

E3 E8 E21 E23 Grinding

5.4 4.7 2.3 1.9 0.6

6.9 6.1 3.0 2.3 1.2

9.1 8.1 3.7 3.0 1.5

13.5 10.6 4.7 4.0 2.3

24.1 20.9 8.7 7.0 2.8

0.46 0.41 0.20 0.15 0.08

0.15 0.13 0.062 0.050 0.025

0.090 0.071 0.032 0.027 0.015

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Fig. 4. Cross-sectioned view SE micrographs of (a) rough EDM’ed WC10Co(Cr/V), (b) fine EDM’ed WC10Co, and (c) fine EDM’ed WC6Co(Cr/V).

carbides and decrease when the cemented carbide is surface finished up to a finer EDM cutting regime. It is worth noting, though, that the combined penetration depth remains below 20 ␮m for all sliding combinations after a 10 km wear path under a 15 N contact load, and thus, the recast material was not completely removed, nor the earlier described ‘bulk’ steady state wear regime has been reached. With decreasing HAZ thickness (ca. 10 ␮m for regime E8) or amount of recast (less than 5 ␮m for E21 and E23), however, the wear depth increases at lower rate towards the ‘bulk’ regime, repre-

sented by the shattered lines in Fig. 3(a). The combined penetration depth for samples with E8 surface finish is noticed to exceed 10 ␮m after a 10 km wear path, which allows to infer that the recast material was completely removed. Indeed, the slope of the penetration depths for the WC12Co(Cr) grade with E8, E21 and E23 surface finish is demonstrated to be approximately equal, and thus, is assumed to represent these of the pristine material. In order to examine the reciprocating sliding wear behavior, research was mainly focused on analysis of wear characteristics

Table 5 Wear scar dimensions for wire-EDM’ed and ground cemented carbide flat/WC6Co(Cr/V) pin sliding combinations (FN = 15 N, v = 0.3 m/s, s = 10 km) Grade

Surface finishing

Width (mm)

Depth (␮m)

Vwear (10−3 mm3 )

kV (10−6 mm3 N−1 m−1 )

WC10Co

E3 E8 E21 E23 Grinding

1.00 0.90 0.85 0.80 0.75

10.9 6.6 4.9 4.1 3.0

40.9 25.1 11.7 7.9 2.8

0.27 0.17 0.078 0.053 0.019

WC12Co(V)

E3 E8 E21 E23 Grinding

0.95 0.80 0.75 0.70 0.65

9.7 6.1 3.4 2.7 1.6

37.2 23.6 10.6 6.3 1.3

0.25 0.16 0.071 0.042 0.0086

WC12Co(Cr)

E3 E8 E21 E23 Grinding

1.00 0.85 0.80 0.75 0.65

9.9 6.1 3.1 2.6 1.7

38.9 25.0 11.1 6.2 1.4

0.26 0.17 0.074 0.041 0.0093

WC10Co(Cr/V)

E3 E8 E21 E23 Grinding

0.95 0.80 0.75 0.65 0.50

6.2 3.3 2.3 1.7 0.9

25.8 9.9 6.5 4.9 0.7

0.17 0.066 0.043 0.033 0.0046

WC6Co(Cr/V)

E3 E8 E21 E23 Grinding

0.95 0.75 0.70 0.70 0.60

6.2 3.6 2.6 2.2 0.9

30.4 16.8 9.4 6.0 0.8

0.20 0.11 0.063 0.040 0.0052

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Fig. 5. Cross-sectioned wear tracks after sliding 10 km at 0.3 m/s under 15 N against WC6Co(Cr/V) pins for WC10Co grade with ground (a) and EDM-finishing regimes E3 (b), E21 (c and d) and E3 (e and f).

in the steady state regime. Wear experiments were therefore conducted up to a total sliding distance of 10 km. The obtained 3D wear track surface topographies allow to determine the corresponding wear track volumes (Vwear ). Based on these results, a post-mortem volumetric wear rate (kV ) can be derived according to Eq. (4): kV =

Vwear (mm3 N−1 m−1 ) FN · s

of the pin, which was not taken into account during post-mortem quantification as it appeared to be too small to be quantified accurately.

3.3. Wear surface analysis

(4)

The post-mortem obtained wear track dimensions, originating from the sliding wear tests described in Fig. 3, together with the corresponding volumetric wear rates (kV ) are summarised in Table 5 for the distinctive cemented carbides. Under identical conditions of 10 km sliding distance, 0.3 m s−1 sliding speed and 15 N contact load, the volumetric wear rates are noticed to vary between 4.6 × 10−9 and 2.7 × 10−7 mm3 N−1 m−1 . Consistent with the online recorded results shown in Table 4, the smallest depth and width of the wear scars are measured for the ground samples of WC10Co(Cr/V) cemented carbides, whereas the highest values are found with the rough cut EDM WC10Co grade specimens. Comparing the post-mortem, i.e., obtained by surface profilometry (see Table 5), with the online recorded, i.e., from the pin displacement (see Table 4), wear depth after 10 km of sliding distance reveals small deviations. This is mostly attributed to the wear

From the recorded tribological responses of the cemented carbides with distinctive wire-EDM surface finishing conditions, it can be inferred that the thickness of the recast layer primary determines the wear damage and wear rate, particularly in the initial phase of wear testing. The wear resistance of the recast material, in which droplets, voids and cracks are generated, Fig. 4, is substantially lower than that of the cemented carbide material, as could be deduced from the higher wear rate of the rough cut EDM (regime E3) compared to the ground material. Indeed, the rough EDM specimens exhibit higher surface roughness, compared to the finer EDM regimes and grinding operation, and thus, cause more pronounced abrasion owing to an increased ploughing component [18–22]. Furthermore, the wire-EDM induced heat-affected zone was found to contain surface cracks, penetrating into the base material, Fig. 4(a), and/or thermal cracks of WC grains, Fig. 4(b), both after rough and finer EDM cutting. These already existing transgranular and/or

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Fig. 6. Top view SE and BSE micrographs of wear tracks after sliding WC6Co(Cr/V) pins 10 km at 0.3 m/s under 15 N against WC10Co(Cr/V) grade with ground (a) and EDM-finishing regimes E21 (b–d) and E3 (e–h).

intergranular cracks serve as notches for the (sub)surface material during the reciprocating sliding wear tests. Growth and propagation of these cracks increases the probability of grain fracture (see for instance Figs. 9(d) and 11(d)). In addition, Co binder depletion and concomitant loosening of WC grains after the EDM process was discovered, Fig. 4(c), resulting in a considerable deterioration of strength properties at the surface [4]. The wear mechanisms were investigated by analysing the wear scars by scanning electron microscopy. Within the range of the regarded surface finishing conditions, the optical appearance of the wear track was smooth, indicating that the surface of the cemented

carbide was burnished as a result of the sliding contact with the pin. This phenomenon is exemplified by Table 6, in which the Ra and Rt surface roughness before and after wear testing are compared for a rough cut wire-EDM (regime E3) WC12Co(Cr) grade and various wear paths. It is clear that the normal roughness profile, measured in the wear track of the cemented carbide, yields lower Ra - and Rt values compared to the corresponding surface roughness for the original wire-EDM’ed WC12Co(Cr) grade. Furthermore, it can be seen that the roughness diminishes strongly during the first sliding meters, and thus, the polishing effect is found to occur particularly during the running-in stage of the wear process.

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Fig. 7. Cross-sectioned wear tracks after sliding WC6Co(Cr/V) pins 10 km at 0.3 m/s under 15 N against WC10Co(V) grade with ground (a) and EDM-finishing regime E3 (b) and E21 (c and d).

The results of SEM analysis on the central part of wear scars originating from sliding wear tests at 0.3 m s−1 and under a 15 N contact load on distinctive ground and wire-EDM’ed cemented carbides are presented in Figs. 5–8. Cross-sectioned views of wear tracks on ground and wire-EDM’ed WC10Co cemented carbides are compared in Fig. 5. The impact of polishing is illustrated in Fig. 5(b), in which the wear track can clearly be distinguished from the original surface, also demonstrating the procedure of how the wear tracks are located to be examined in more detail. Polishing of WC grains is accompanied by other emerging wear mechanisms as well. The general wear process of WC-Co cemented carbides was found to occur by the accumulation of damage, fracture and removal of WC grains [23–25]. According to previous research [7,8], the primary wear mechanisms for the ground specimens appear to be surface Co binder removal, WC grain cracking and WC grain pull out, as illustrated by Fig. 5(a). These wear mechanisms were also found on wire-EDM’ed surfaces, however to a higher degree and different mutual relative importance, owing to the thermal impact of the EDM process, such as depletion of Co binder, which is clearly visible when comparing SE and BSE micrographs in Fig. 5(c) and (d). Moreover, a small amount of recast layer is still present on rough cut EDM surfaces, despite the sliding contact for a 10 km wear path, as can be seen in Fig. 5(e) and (f). The microcracks, observed in the wear tracks of both ground and wire-EDM’ed cemented carbides, are induced by tangential stresses caused by the sliding pin-on-flat contact. Similar trends are observed in top views of the central part of the wear tracks on ground and wire-EDM’ed WC10Co(Cr/V) cemented

carbides, Fig. 6. The impact of polishing allowed to distinguish the wear track from the original surface finished area, Fig. 6(b) and (e). As for the ground specimen, Fig. 6(a), the wear surface is covered with small wear debris [26], occurring as bright spots, whereas the microstructure corresponds to the base material, i.e., the grain size of the WC phase is not changed due to the sliding of the pin. Similar observations are made for the fine cut (EDM regime E21) sample, exhibiting islandized areas of recast material on the surface, Fig. 6(c) and (d). Moreover, surface cracks are observed to be mainly intergranular, Fig. 6(d), due to the fine grain size distribution of this grade. These cracks are more likely to occur at wire-EDM’ed surfaces as a result of the already existing microcracks serving as notches for the (sub)surface material during the reciprocal sliding movement of the cemented carbide pin. The wear surface of rough cut (EDM regime E3) WC10Co(Cr/V) cemented carbide, is noticed to display recast material for the most part, characterised by voids and a randomly distributed microcrack network, whereas the original base material occurs only occasionally, Fig. 6(f)–(h). Cross-sectioned views on the central part of the wear tracks on ground and wire-EDM’ed WC12Co(V) cemented carbides are compared in Fig. 7, confirming the presence of recast material on the wear surface of rough cut (EDM regime E3), contrary to the fine cut (EDM regime E21) and ground specimens. Comparing the SEmode and BSE-mode SEM micrographs in Fig. 7(c) and (d) reveals the occurrence of Co binder depletion as a result of the thermal impact during the EDM process.

Table 6 Ra and Rt surface roughness for the rough cut (regime E3) WC12Co(Cr) grade before and after wear testing as function of sliding distance (s) Roughness

Ra (␮m)

s [km]

0

0.1

1

4

10

0

0.1

1

4

10

E3

2.37

0.14

0.088

0.071

0.078

16.08

1.86

1.30

1.02

1.06

Rt (␮m)

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Fig. 8. Surface views of WC12Co(Cr): microstructure (a) and central wear track on ground (b) and rough EDM’ed (c and d) surface, after sliding WC6Co(Cr/V) pins for 10 km at 0.3 m/s under 15 N.

Top views of a ground WC12Co(Cr) cemented carbide before and after wear testing, as well as the central parts of wear tracks on rough cut (EDM regime E3) WC12Co(Cr) cemented carbides, are compared in Fig. 8. The polishing impact clearly locates the position of the wear track on the surface, Fig. 8(c). Comparing Fig. 8(a) and (b) reveals that the wear surface is covered by small wear debris particles, occurring as bright spots in Fig. 8(b), whereas the microstructure in the wear scar corresponds to the microstructure of the base material. As for the wire-EDM surface, the recast layer on the rough cut wire-EDM cemented carbide turns out to be not

completely removed by the sliding wear experiment, Fig. 8(c) and (d). Microcracks and wear debris are observed on the wear surface. Comparing the wear tracks on ground and wire-EDM’ed surfaces allows to infer that the observed Co binder depletion (see Figs. 5(c) and (d) and 7(c) and (d)) as well as Co binder modification (see the lighter occurring Co phase in Fig. 5(e) and (f)) are not caused by the thermal input during the reciprocative sliding wear process, and thus, represent exclusive remainders of the EDM-finishing operation, since these phenomena were never observed for wear scars on ground cemented carbides.

Fig. 9. Wear volume (a) and wear rate (b) as function of original surface roughness of WC-Co cemented carbides, slid for 10 km at 0.3 m/s under 15 N against WC6Co(Cr/V) pins.

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Fig. 10. Wear volume (a) and volumetric wear rate (b) as function of heat-affected zone thickness of WC-Co cemented carbides, slid for 10 km at 0.3 m/s under 15 N against WC6Co(Cr/V) pins.

3.4. Influence of surface conditions The impact of surface roughness, obtained by grinding and wireEDM, on wear volume (Vwear ) and volumetric wear rate (kV ) is investigated in Fig. 9 for all cemented carbides after reciprocative sliding wear experiments using a 15 N contact load and a 10 km wear path. Within the range of Ra and Rt , both the wear volume and wear rate are noticed to decrease with enhanced surface finish refinement. The rough cut EDM cemented carbides exhibit the highest wear level, which improves drastically by the execution of gradually finer EDM-finishing cuts. Comparing the wear volumes for the finest EDM regimes with ground surfaces, both exhibiting similar roughness levels, reveals that the ground surfaces yield better wear resistance, in full agreement with the results presented in Fig. 3. The influence of EDM is even more obvious when wear volume and wear rate are plotted against the thickness of the wire-EDM induced HAZ, Fig. 10. The thickness of the heat-affected zone was derived from cross-sectioned SEM views of wire-EDM’ed cemented

Fig. 11. Surface residual stress in the WC phase of 2 cemented carbides after grinding and EDM finishing (regime E23): a + value indicates a tensile stress, whereas a − value represents a compressive stress.

carbides. It is clear that no recast layer appears with the ground specimens, yielding better results regarding wear resistance. The higher wear level for wire-EDM’ed cemented carbides can therefore be fully ascribed to the thermally induced heat-affected zone and a recast layer, in which a resolidified molten material, voids and residual tensile surface stresses are generated, resulting in microcracks in the wire-EDM surface layer. As a result, the relative importance of emerging wear mechanisms, participating in the global surface damage, and the tribological compatibility with the WC6Co(Cr/V) pin, are modified, when compared to the ground cemented carbides, where the pin makes direct contact with the original base material. The inferior strength and wear resistance of the heat-affected material can be explained in terms of stress state modification. After cooling down the WC-Co based cemented carbide from sintering, the WC phase is in compression, whereas the Co binder phase is in tension, due to the higher thermal expansion coefficient of Co compared to WC. Contrary to the compressive stress state in the bulk material, the wire-EDM process was found to generate residual surface stresses which are tensile in nature. This is illustrated by Fig. 11, in which the surface residual stresses in the WC phase of the cemented carbide grades after grinding and the finest EDM-finishing cut (regime E23) are compared. The measurements were executed by X-ray diffraction on a Siemens D500 XRD, using the d–sin2 method. The (3 0 0) WC peak, corresponding with a diffraction angle 2 = 133.31◦ was applied in order to acquire the residual stress. The sin2 range was varied from 0 to 0.6 in steps of 0.1, and the angle 2 was varied between 130◦ and 136◦ at 0.02◦ /steps of 5 s. It should be clear that a significant compressive stress is present in the ground materials, owing to the mechanical impact during grinding, whereas a tensile stress is measured on the EDM surfaces, as a result of the thermal impact. Indeed, during the process of EDM, a little melted material resolidifies to become many droplets in clusters, which are distributed on the EDM’ed surface. At the same time, the material on the surface shrinks in the process of resolidification due to the dielectric cooling. Some regions, where the melted material is resolidified later on, do not have enough melted material to fill voids, which are induced by the shrinkage. Thus, a cavity is left on the wireEDM’ed surface. On the other hand, residual tensile stress occurs in the recast layer because of material shrinkage, which results in

K. Bonny et al. / Wear 266 (2009) 84–95

a network of microcracks mostly passing through cavities in the EDM’ed surface. During the sliding wear experiments, the tangential stresses will build up with the residual tensile stresses in the wire-EDM surface, whereas the compressive stress state of ground surfaces more or less eliminates the tangential contact stress. 4. Conclusions Dry reciprocative sliding experiments on WC-Co/WC-Co cemented carbide combinations revealed that the impact of surface finishing operations on wear performance is quite pronounced. The detrimental effect of wire-EDM on wear resistance could be attributed to a thermally induced recast layer and the concomitant reduced surface quality, i.e., binder depletion, residual tensile surface stresses, surface WC grain cracking and higher surface roughness, each invigorating the observed wear mechanisms to a certain degree, i.e., grain polishing, abrasion, microcracking, surface binder removal, grain cracking and grain pull out. The thickness of the recast layer primary determines the running-in wear. However, consecutive execution of gradually finer EDM-finishing regimes reduces the wear rate considerably, down to values nearby those for equivalent ground specimens. The volumetric wear rate for a 10 km wear path using a 15 N contact load and a 0.3 m s−1 sliding speed varied in the range of 4.6 × 10−9 and 2.7 × 10−7 mm3 N−1 m−1 . Acknowledgements This work was co-financed with a research fellowship of the Flemish Institute for the promotion of Innovation by Science and Technology in industry (IWT) under project contract number GBOU-IWT-010071-SPARK. The authors gratefully recognize all the support, scientific contributions and stimulating collaboration from the partners from the University of Ghent (UGent) and the Catholic University of Leuven (K.U. Leuven). Special acknowledgement goes to CERATIZIT for supplying the hardmetal grades and pins. References [1] J. Kozak, K.P. Rajurkar, N. Chandarana, Machining of low electrical conductive materials by wire electrical discharge machining (WEDM), J. Mater. Process. Technol. 149 (1–3) (2004) 266–271. [2] A.M. Gadalla, W. Tsai, Machining of WC-Co composites, Mater. Manuf. Process. 4 (1989) 411–423. [3] A.M. Gadalla, W. Tsai, Electrical discharge machining of tungsten carbide-cobalt composites, J. Am. Ceram. Soc. 72 (1989) 1396–1401.

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